Radio Frequency IDentification (RFID) is a communication technology, which can identify a specific target and read the corresponding data via a wireless signal without a mechanical or optical contact between an identifying system and the specific target. As the wireless RFID technology became more popular, many hand held devices are installed with RFID antenna.
RFID tag antenna can be categorized into, according to its operating frequencies, low-frequency antenna, high-frequency antenna, ultra-high frequency antenna, and microwave antenna. The low-frequency antenna is operating within the range of 125 kHz to 134 kHz; the high frequency antenna is operating at 13.56 MHz; the ultra-high frequency antenna is operating within the range of 840 MHz˜960 MHz; and microwave antenna is operating within the range of 2.45 GHz˜5.8 GHz. Generally speaking, the wireless RFID tag antenna installed in the hand held electronic apparatus belongs to an ultra-high frequency antenna, and the ultra-high frequency antenna uses radiation for its transmission.
FIG. 1A illustrates a conventional RFID tag 10 and its impedance match method. As shown in FIG. 1A, the conventional RFID tag 10 comprises an antenna body 101 and a couple loop 102, which disposed on a substrate 103 with two feed-in points 102a/102b. For the reason of impedance match, the couple loop 102 can be deemed as a small-scale inductor, barely having any radiation function. Alternatively, in order to achieve the impedance match between the antenna and its chip, inevitably, the small-scaled loop 102, barely having any radiation function, (the small couple loop 102 is usually smaller than 30% of the size of the antenna body 101) so as to ensure the input impedance for the antenna has appropriate inductive reactance and accordingly the capacitive reactance of the chip can be eliminated and further a conjugate match can be achieved. In the case of the match, the return loss of the input port can be illustrated in FIG. 1B, and its impedance characteristics can be further illustrated in FIG. 2 (where the real part of the input impedance of the antenna is denoted as Ra, the imaginary part is denoted as Xa; and, the real part of the input impedance of the chip is denoted as Rc, and the imaginary part is denoted as Xc)
Accordingly, the conventional RFIG tag antenna is easy interfered by the external noise and factor, resulting in it is hard to reduce the size of the antenna to design in the small-scale electronic products.